1. Field of the Invention
The present invention relates to a method of making semiconductor devices, and more particularly to a method of leveling the laminated surface of a semi-conductor substrate, thereby providing a leveled surface on which wiring conductors are to be attached.
2. Description of the Prior Art
One example of such leveling method is described with reference to FIG. 2a and 2b. As seen from FIG. 2a, a silicon substrate 1 has aluminum wiring conductors 2 of different widths attached to its top surface. A relatively thick insulating film 10 is formed on the surface of the semiconductor substrate 1. Finally, a coating layer 11 is formed on the insulating film 10. The lamination o the silicon substrate 1 is subjected to ion etching, which is carefully conditioned so as to permit the etching away of the different materials of the insulating and coating layers at a same rate. FIG. 2b shows a section of the insulating film 10 thus leveled. This leveling method has following defects: first, the insulating film must be very thick, and thick enough to prevent undesired exposure of aluminum wiring conductors 2 when the insulating film is etched out deeply at selected areas because the etching rate is liable to vary from place to place; second, the overlying coating layer must be very thick because otherwise, its contour would be similar to the underlying irregular contour of the insulating film 10, thus allowing uneven etching-out of the lamination on the substrate; third, the insulating film 10 remains so thick over aluminum conductors 2 that contact or through holes when made deep enough to reach such aluminum conductors 2 will be inevitably deep and large in size, thus sometimes preventing lead wires from reaching underlying conductors, and causing underside space between lead wires and surrounding hole to allow invasion of moisture in contact regions; fourth, the etching away of the two different materials at same rate is very difficult; and fifth, the critical point at which the etching away of the lamination must be stopped cannot be determined with accuracy, and therefore, it is most likely that the etching away is stopped at a premature stage with the result that the leveling is remote from satisfaction.
Another examples of levelling method are illustrated in FIG. 3a and 3b. As seen from FIG. 3a, a silicon substrate 1 has aluminum wiring conductors 2 attached to its surface and a relatively thick coating layer 11 formed on the surface of the semiconductor substrate 1 and the aluminum wiring conductors 2 thereon. Lest the coating layer 11 should be influenced by the underlying irregularity, the coating layer 11 must be thick enough for its upper surface to become substantially flat. The coating layer 11 is used as insulating layer to which another aluminum wiring conductors are to be attached. This insulating layer 11, however, is porous, allowing invasion of moisture to deteriorate the quality of insulation.
In an attempt to avoid this the porous layer 11 is sandwiched between lower and upper insulation films 12A and 12B. This, however, makes it difficult to make through holes, and the porous inner surface of the through hole surrounding the lead wire is exposed to the atmosphere, releasing the remaining volatile substance or moisture from the porous inner surface with the result that the quality of wiring connection is lowered.